1. Field of the Invention
This invention relates to optical fibre couplers.
2. Description of Related Art
Optical fibre couplers may be constructed so that there are at least two uni-directional ports (one carrying incoming light, one other carrying outgoing light), one bidirectional port and at least one unused port which is terminated. If the bidirectional port is also terminated, the isolation of the coupler may be measured between the unidirectional ports.
In certain optical fibre applications, the isolation is required to be high; this isolation is governed by reflections at the unused port or ports. Whilst each port can be terminated to reduce reflections using a matching refractive index medium placed outside the coupler, it is preferable that each termination is inside the coupler.
Thus the unused port of a 2.times.2 coupler may be terminated to form a 1.times.2 coupler. Similarly 1.times.3 and 1.times.4 couplers may be formed by terminating all but one of the constituent fibres.
It is known to reduce reflections by terminating the unused port outside the coupler by embedding a tapered end of the respective fibre in a matching refractive index medium.
However, the tapering of the fibre after the coupler is formed has been found to weaken or alter the performance of the coupler.